1. Field of the Invention
This invention relates to the measurement of the speed of a traveling looped belt. More specifically, this invention relates to the measurement of the speed of a somewhat elastic belt, such as a papermaking felt, and to the accurate determination of the rotational position of specific portions of such a felt. Still more specifically, this invention relates to the measurement of the cyclic speed and rotational position of a papermaking felt by producing an area of discontinuity, or caliper variation, in the surface of the felt and bringing the discontinuity into spaced adjacency with a source of fluid pressure and means for measuring changes in the fluid pressure near the surface of the felt as the discontinuity passes in close proximity with the means for measuring fluid pressure changes.
2. Description of the Prior Art
In papermaking machines, looped felts are used to dewater the paper web being formed by passing with the web through a nip between two rolls. The water expressed from the web is received by the felt in the nip and removed subsequently by other equipment, which may include the press rolls. Felts are also used in the dryer section of a papermaking machine, but they typically do not pass through nipped rolls.
Regardless of where felts are used in papermaking machines, they become worn with use over a period of time. They also tend to stretch with both the absorption of water and wear. Such wear tends to produce irregularities in the felt, such as corrugations.
In modern papermaking machines, with speeds commonly about 1,000-1,500 meters per minute, and design speeds even higher, it is important that all sources of vibration and roll bounce be eliminated, or at least diminished, and all aspects of felt condition be monitored to ensure the production of a quality sheet of paper at high speeds. The production of a quality paper sheet at relatively low speeds can be accomplished without encountering, must less recognizing, many of the problems which arise when the same quality paper is required at increased production speeds. In today's marketplace, the production of a quality product at high speeds is essential to be economically competitive.
In order to produce a quality paper sheet product at high speeds, the force between nipped rolls against the web and felt passing therebetween must be increased. As the felt wears, corrugations develop in it which produce vibrations which tend to produce imperfections in the wet paper web as it is being processed. In addition, such vibration over time tends to create imperfections in the covers of the rolls nipping the felt and web. Damaged roll covers have to be replaced more frequently. The felt corrugations and roll cover imperfections exacerbate the imperfections and non-uniformities in the paper web being produced and the frequency of their creation.
It is therefore of great importance that machine runnability be maintained at a uniformly high level; that timely maintenance be conducted on the various components of the papermaking machine, such as felt replacement and roll cover conditioning, and overall optimization of the quality operation of the papermaking machine from the headbox to the reel be maintained at a high level.
Accordingly, felt wear and deterioration, and their effect on vibration and roll bounce, are important operating parameters necessary to predict maintenance of the papermaking machine components and to correlate other operating parameters, such as headbox pulsations, comparing basis weight measurements of the paper sheet at the reel and controlling pressure surges in the forming area of the papermaking machine.
For example, if the felt is becoming corrugated to a slight extent which is not detectable by the human eye, but which nevertheless is affecting water removal, machine direction caliper control, or nip vibration, in the press section, the press nip, or nips, can be adjusted slightly, or the machine speed can be increased or decreased slightly, or the angle at which the felt passes through the nip can be changed slightly to eliminate or, at least, mitigate variations in the caliper, the extent of nip bounce, water removal in the press nips and the rate and location of water removal and stock surges in the forming section. These are only some of the adjustments which can be made to produce a more uniform, high quality paper sheet.
In order to both accurately determine the source and extent of such imperfections, variations and non-uniformities in the paper product, it is very important that the cyclic speed of the somewhat elastic and deformable felt be measured accurately. It is also important to know the exact part of the felt which is passing through a press nip when instruments operatively linked to the nipped rolls indicate an undesirable phenomenon is occurring, such as roll bounce or nip vibration. In the past, attempts to measure the felt speed by measuring the rotational speed of the rolls, such as press and felt rolls, contacting the felt have not proved reliable. The elasticity, floppiness and deformability of the web simply made determinations of its speed based on the rotational speed of the rolls coming into contact with the felt inaccurate to the extent necessary to make calculations necessary to adjust other operating parameters. Further, merely measuring roll speed is ineffective in trying to identify with precision the exact part of the felt passing through a nip when vibration occurs.
Prior methods of measuring the web cyclic speed also included optical methods which utilized a mark on the surface of the felt which was read by an optical scanner. As the felt became more worn and filled with contaminants, such as fines, pulp fibers, pitch, ink, clay and dirt, the demarkation between the mark on the felt and the unmarked portion of the felt became blurred and the optical scanners could not provide accurate indications of when, to the required time tolerances, the mark passed the scanner.